The molecular nature of the treosulphan and DEB-induced mutants in AS52 (CHO cells) in vitro were investigated to ascertain whether both chemicals were mutagenic via the same ultimate mutagen. PCR analyses of restriction fragments from treosulphan and DEB-induced mutants showed similar patterns; 23% and 24% of the mutants carried point mutations, 26% and 21% had partial deletions of the targeted chromosome region, and 52% and 55% had complete deletions of this region, for treosulphan and DEB, respectively. These results suggest that the same molecular moiety is responsible for the mutagenicity of both chemicals. The point mutations were sequenced. There were no significant differences in the types of base changes induced by treosulphan and DEB. A number of metal salts were tested for their ability to mutate AS52 cells in culture. Mercuric chloride, zinc chloride, nickel sulfate, cadmium chloride, and ferrous sulfate were mutagenic; magnesium sulfate and manganese sulfate were toxic to the cells, but not mutagenic. The laboratory portions of these studies have been terminated. Efforts are concentrated on the analyses of the data and preparation of publications describing the results.